Google documents lets you copy the formatting you’ve applied to a specific section of text to another section using the paint format tool. If you’re familiar with the paint format tool in Google spreadsheets, this works in a similar manner.
To use this tool, select the text that's formatted in the way that you want to copy. Then, click the paintbrush icon in your toolbar, and select the text to which you want to apply the formatting. The formatting from the original text will be copied to the selected text.
To change the formatting in multiple places within your document, double-click the paintbrush icon. You'll enter a mode that lets you highlight multiple text selections, and apply the same formatting to each selection. When you're done applying formatting to these selections, click the paintbrush icon again.
You can also use the paint format tool with keyboard shortcuts. Ctrl+Option+C will copy the formatting of the text you've selected, and Ctrl+Option+V will apply any copy that formatting to a different text selection.